1. Field of the Invention
The present invention relates to a method for the sintering treatment of cemented carbides and a sintering furnace used for the production of sintered hard alloys by powder metallurgy of cemented carbides consisting mainly of tungsten carbide, cermets consisting mainly of titanium carbide, ceramics consisting mainly of aluminum oxide, etc.
2. Description of the Prior Art
For the production of these alloys, there have hitherto been proposed a method comprising preparing a fine powder pulverized and mixed sufficiently in a desired proportion, subjecting to pressing and sintering the pressed body or compact as it is and a method comprising pre-sintering a pressed body or compact, subjecting to forming and then carrying out a final sintering. The above described fine powder is ordinarily mixed with a lubricant so as to decrease the friction among the powder particles or between the powder and a mold in a pressing operation, thereby raising the dimensional precision and preventing a sliding defect. Paraffin, wax, camphor and the like are used as such a lubricant. Since these lubricants are organic materials, however, it is necessary to install a pre-sintering furnace or to provide a sintering furnace with a means for removing the lubricant so that a product alloy is not affected by the carbon contained therein. Furthermore, it is necessary to provide such a furnace structure that such a lubricant oil is removed from a pressed body or compact at a temperature of as low as possible and the removed lubricant is not retained, deposited and accumulated in or on a heat treatment chamber, furnace wall, exhaust system, etc. The sintering furnace of the prior art, however, has not a suitable structure as a means for removing a lubricant and thus is not freed from the disadvantage that the lubricant is deposited and accumulated on the heat insulators, inner walls, pipes and valves of the exhaust system in the heat treatment chamber and the accumulated lubricant must be removed and swept, which requires much time and cost, resulting in lowering of the operating ratio of the furnace.
In general, unless the sintering temperature of a sintered hard alloy is sufficiently high, for example, 1200.degree. C. or higher, a high density and high grade alloy cannot be obtained and in order to obtain a desired quality, it is important to select suitably an atmosphere during pre-sintering, sintering and cooling. When a lubricant is removed followed by raising the temperature and sintering, the lubricant adhered to the furnace wall, etc. contaminates the atmosphere in the furnace and, accordingly, it is difficult to obtain a product with a high grade. When the processings such as presintering, sintering and cooling are continuously carried out in different atmospheres such as gaseous atmospheres and vacuum in the same furnace, a workpiece to be processed is usually moved to different processing positions at a high temperature and it is difficult to move a table carrying the workpiece precisely, for example, by revolving rollers or by moving a forklift since the roller or lift arm is often strained, fatigued or broken by thermal stress.